Longitudinal girder as part of a load bearing structure of a motor vehicle

ABSTRACT

The invention relates to a longitudinal girder used as part of a load bearing structure of a motor vehicle. In order to optimize the flexural strength, load bearing capacity and crash behavior of a longitudinal girder within a given construction area, the inventive longitudinal girder consists of two hollow profiles ( 1   a,    1   b,    2   a,    2   b ) which are disposed at a height/width ratio of approximately 1 and which are connected to each other on both ends thereof ( 5, 6, 9   a,    9   b,    10   a,    10   b ) in a dimensionally stable manner.

The invention relates to a longitudinal member as part of a supportingstructure of a vehicle, with said longitudinal member's width/heightratio being a₀/b₀<1, wherein said longitudinal member comprises at leasttwo hollow sections which extend essentially parallel to each other.

Longitudinal members which have been constructed according to today'sstate of the art have already been optimize with respect to achievinggreat weight savings while at the same time achieving maximum rigidity.In other words, it is probably no longer possible to realise any weightsavings potential purely by reducing the sheet metal thickness or purelyby reducing the cross-sectional area of the profile of the member. Anyfurther reduction in sheet metal thickness would result in inadequaterigidity, in particular in relation to resistance to bending orresistance to buckling, of the longitudinal member.

In order to nevertheless achieve a reduction in weight by reducing thesheet metal thickness, a well-known design measure consists of using ahigher-strength material which provides still adequate mechanicalstrength even at reduced sheet metal thickness so as to maintainadequate strength of the longitudinal member rather than letting thestrength drop to below a specified minimum. The known measure ofproducing the section of the longitudinal member from a high-strengthsteel material and of reducing the sheet metal thickness accordinglywould at first seem to be a possible solution.

However, the use of high-strength steel materials is associated with newproblems in relation to the crash behaviour of the supporting structure.The force F, which in the case of a crash is transferred to thepassenger compartment, is calculated according to the equation F=σ×A,with a designating the apparent limit of elasticity of the material, andwith A=U×t applying, i.e. A is the cross-sectional area of the materialof the section of the longitudinal member with a circumference U and asheet metal thickness t. Since in the case of materials comprisinghigh-strength steel the value σ clearly exceeds the value of materialsin conventional longitudinal members, without a reduction in the sheetmetal thickness t, the extent of force F transferred to the passengercompartment in the case of a crash is clearly unacceptably great(endangering the passengers). On the other hand, if the sheet metalthickness t were to be reduced in order to reduce the force F whichwould be transferred in the case of a crash, this would result in areduction of resistance to bending, thus increasing the danger of aso-called bending collapse. The term “bending collapse” refers tofailure of the longitudinal member as a result of buckling orcollapsing.

The force F which would be transferred in the case of a crash could alsobe achieved by reducing the circumference U of the section of thelongitudinal member, namely by shortening its edges, but such a solutioncannot be considered since it would result in an excessive reduction inthe rigidity of the longitudinal member, which would lead to a reductionin its support function. A reduction in the edge lengths a₀ and b₀ alsoincreases the danger of a bending collapse, since the section wouldbecome narrower.

Therefore, for the reasons mentioned above, considering that the forcetransferred to the passenger compartment has to be kept to a levelsustainable by the passengers, it is not possible to achieve anysignificant weight reduction in relation to the longitudinal member byreducing the edge length of the section and/or by reducing the sheetmetal thickness.

There is an additional reason why the ratio of edge lengths a₀/b₀ cannotbe reduced any further, namely that there are so-called stabilitycriteria, (Schriever, T: “Zur nichtlinearen FE-Analyse desVerformungsverhaltens von Fahrzeuglängsträgern mit gezielt eingebrachtengeometrischen Imperfektionen” [Non-linear FE analysis of the deformationbehaviour of longitudinal members in vehicles, which members comprisepurposefully-created geometric imperfections]; Institut fürKraftfahrwesen Aachen [institute of automotive engineering] RWTH Aachen,Aachen 1990)) in relation to the longitudinal member folding duringdeformation as a result of a crash, which stability criteria have to bemet. A first stability criterion, designated “folding compatibility”demands that, during deformation of the longitudinal member as a resultof a crash, the long sides b₀ of the section must be able to foldwithout any obstruction or hindrance because only in this way can themaximum possible quantity of energy be absorbed and converted todeformation energy. It is therefore not permissible for the interiorsurfaces of the section to touch each other along their longitudinaledges b₀ during folding, as this would impede unrestricted foldformation. For this reason, the longitudinal ratio a₀/b₀ must not bebelow a lower limiting value.

So-called “compactness” is a second stability criterion. A stable andregular folding process as a result of crash deformation depends on theratio of sheet metal thickness t to the long side b₀ of the section ofthe longitudinal member. This ratio t/b₀ must therefore not drop below aspecific critical value. The precise definition of this critical valuedepends among other factors on the type and quality or grade of thematerial of said longitudinal member. It is thus evident that thestability criterion of “compactness”, which the expert endeavours tomeet in the interests of a stable and regular folding process in thecase of a crash, prevents the expert from further reducing the sheetmetal thickness t in an attempt to save weight.

The stability criterion of “folding compatibility” is also not being metin a known longitudinal member of the type mentioned in the introduction(U.S. Pat. No. 4,986,597). In this member, two hollow sections areconnected with flanges along their entire length, in that the hollowsections with the flanges form a further hollow section between eachother. Such a longitudinal member is a functional structural unit, i.e.its two hollow sections, due to their connection along their entirelength, are exposed to loads as a unit, in particular in relation tobending and torsional loads. In this known longitudinal member, theratios of width to height of the individual hollow sections regularlyexceed 1, sometimes even by a significant factor. Such a longitudinalmember therefore is associated with an inherent risk of free foldformation being impeded in the case of a crash, in that the folds offacing sides abut against each other.

Apart from such longitudinal members whose hollow sections constitute afunctional structural unit, a longitudinal member structure is knownwhich comprises a double longitudinal member plane, each made fromcompact individual sections arranged so as to be spaced apart from eachother (Stahl und Eisen [steel and iron] 121 (2001), no. 7, pages 36 and37). In such a structure of a longitudinal member, in the case of afrontal crash and also during simple bending loads, the individualsections, which are hollow sections, are exposed to loads independentlyof each other, because, in contrast to the situation existing in thecontext of the above-mentioned longitudinal member, no alternatingsupport can be provided because the individual sections are notinterconnected. This means that each individual section has to bedesigned to withstand the maximum load that can be exerted. This isusually implemented by means of increased wall thickness of theindividual sections. This known longitudinal member with a double planeis therefore different from a supporting element whose hollow sectionsconstitute a functional structural unit.

It is thus the object of the invention to provide a longitudinal memberfor vehicles, in a light-weight design, which has the followingcharacteristics:

-   -   a) The weight of the longitudinal member is less than that of        longitudinal members made according to the conventional design        principle.    -   b) The rigidity of the longitudinal member, in particular its        resistance to bending, is at least equal to that in longitudinal        members made according to the conventional design principle.    -   c) The longitudinal member's energy absorption capacity in the        case of a crash exceeds that of longitudinal members made        according to the conventional design principle.    -   d) The longitudinal member does not take up any more design        space than longitudinal members made according to the        conventional design principle.

The term “longitudinal member made according to the conventional designprinciple” refers to a longitudinal member which comprises a closedsection of a height b₀ which clearly exceeds its width a₀, as a rule bya factor of 2-3. The closed profile can comprise two section halvesconnected to each other by joining-flanges (so-called monocoqueconstruction), or it can be a closed section without a flange. In anycase, the geometric condition of the “height b₀ exceeding the width a₀”is a characteristic condition. Conventional longitudinal members are ofsuch geometric shape because of the excellent resistance to bendingwhich such a section provides around an axis which is perpendicular toits longitudinal axis.

According to the invention, this object is met by a longitudinal memberof the type mentioned in the introduction, in which longitudinal memberthe hollow sections: have a width/height ratio of a_(1,2)/b_(1,2) 1; arearranged so as to be spaced apart from each other with a free spacein-between; and are interconnected at both ends so as to bedimensionally stable. In particular, the overall height of thelongitudinal member should very substantially exceed the height of eachhollow section.

With external dimensions which are identical to those of a longitudinalmember made according to the conventional design principle, thelongitudinal member according to the invention has a resistance tobending which is at least equal to that of the member made according tothe conventional design principle, so that said longitudinal memberaccording to the invention can fully meet the support function. However,it also meets the stability criteria of “folding compatibility” and“compactness” in that each individual profile meets these criteria. Inthe case of a crash all individual sections can fold without anyhindrance due to this dimensioning. This ensures maximum energyabsorption in the case of a crash. Purely based on the new geometricdesign and dimensioning of the longitudinal member, a weight savingpotential of 20% results.

Plates and/or sheet metal pieces, installed at the ends and/orlaterally, provide dimensionally stable connections between the hollowsections. However, the dimensionally stable connections can also beprovided by other adjoining components.

Below, the invention is explained in more detail with reference to adrawing which shows the following:

FIG. 1 a lateral diagrammatic view as well as the associatedcross-sectional view of a longitudinal member of a supporting structureof a vehicle, comprising two hollow sections which constitute afunctional structural unit;

FIG. 2 a perspective view, including several associated cross-sectionalviews, of two longitudinal members according to FIG. 1, connected to apassenger compartment, as part of a supporting structure of a vehicle inspace-frame design;

FIG. 3 a perspective view, including several associated cross-sectionalviews, of two longitudinal members according to FIG. 1, connected to apassenger compartment, as part of a supporting structure of a vehicle inspace-frame design, in an embodiment which differs from that of FIG. 2;and

FIG. 4 a diagrammatic cross-sectional view of a comparison between alongitudinal member according to the invention and a longitudinal membermade according to the conventional design principle.

The longitudinal member 1, 2, 3, 4 according to the invention forms partof a supporting structure T of a vehicle in space-frame design, withparts of the passenger compartment also being shown in FIGS. 2 and 3.

Each longitudinal member 1, 2, 3, 4 comprises two hollow sections 1 a, 1b, 2 a, 2 b, 3 a, 3 b, 4 a, 4 b made of sheet steel, which extendparallel in relation to each other and are spaced apart from each other.At their ends, both hollow sections 1 a, 1 b, 2 a, 2 b, 3 a, 3 b, 4 a, 4b are interconnected so as to be dimensionally stable, so that they forma functional unit. FIG. 4 diagrammatically shows these dimensionallystable connections as connectors 1 c, 1 d. They can be of variousdesigns, in particular they can be end plates/sheets or lateralplates/sheets. It is also possible for the adjacent support structure Tto be included in the dimensionally stable connections, as shown inFIGS. 2 and 3. All the conventional techniques can be considered asconnection techniques, for example welded or soldered connections, screwconnections, adhesive connections or rivet connections. The decisivepoint is that any such connections interconnect the hollow sections 2, 3in a dimensionally stable manner.

The front ends of the hollow sections 1 a, 1 b, 2 a, 2 b, 3 a, 3 b, 4 a,4 b, as shown in both embodiments of FIGS. 2 and 3, are connected by wayof plates/sheets 5, 6, 7, 8 arranged at the ends. In the embodimentshown in FIG. 2, the rear ends are integrally connected in anoverlapping manner to hollow sections of the supporting structure T ofthe passenger compartment, in particular by way of soldered connectionson the one hand, and are interconnected so as to be dimensionally stableby way of lateral plates/sheets 9 a, 9 b, 10 a, 10 b on the other hand.In the embodiment according to FIG. 3, the rear ends are interconnectedin an overlapping manner to the rigid hollow section of the supportingstructure T of the passenger compartment, and furthermore to each otherso as to be dimensionally stable.

FIG. 4 shows a comparison between a longitudinal member comprising asingle hollow section, made according to the conventional designprinciple, and a longitudinal member according to the invention, whichlatter member comprises two hollow sections 1 a, 1 b which form afunctional structural unit. The conventional longitudinal member hasbeen made in stressed-skin construction, i.e. its two shells 11, 12 areinterconnected by way of flanges 11 a, 11 b, 12 a, 12 b. As thecomparison shows, the external dimensions of the two longitudinalmembers are identical, with the width a₀=a₁=a₂. The overall height ofthe conventional longitudinal member is b₀=b+2c. In the longitudinalmember according to the invention, the overall height is identical,being b₀=b₂+b₂+d, with d=height of the free space between the two hollowsections 1 a, 1 b. Preferably, b₁ b₂ and d=0.5 b₁ to 1.0 b₁. As far asthe height/width ratio is concerned, a₀/b₀<1 applies, in particulara₀/b₀<<1, wherein “<<” denotes a factor of at least “2”, as a rule ofbetween “2” and “3”. At the end of the description, two comparisons forspecific dimensioning have been provided in relation to theselongitudinal members. Example 1 shows the higher-strength steel DP-K34/60 with an elongation limit R_(p0.2)=340 N/mm² as a material for thelongitudinal member according to the invention, while the comparisonlongitudinal member made according to the conventional design principlewas made from ZStE 300 (elongation limit R_(p0.2)=340 N/mm². Whencompared to the conventional longitudinal member, the wall thickness ofthe individual profiles was reduced by approx. 25.93% from 1.35 mm to1.00 mm. The free cut edge A relevant for load transmission through thelongitudinal member (in example 1 referred to as “cross-sectional area”)in the longitudinal member according to the invention is only A=496 mm²,while in the conventional longitudinal member it is A=623.7 mm². Sincethe mass of the longitudinal member is proportional to its free cut edge(mass=p×L×A, with p×L=const.), a comparison of these values determinesthat the longitudinal member according to the invention has only 79.5%of the mass of the conventional longitudinal member, and that thusweight savings of 20.5% were achieved.

The values relating to the force F_(m) show that the quantity of energyabsorbed in the case of a crash is clearly greater in the longitudinalmember according to the invention than in the conventional longitudinalmember. This comparison can be made using the values for F_(m), becausethe absorbed energy is proportional to the force F_(m), which iscalculated from the equation F_(m)=σ_(m)×A, with σ_(m) denoting theaverage tension effective in the longitudinal member$\sigma_{m} = {\left\lbrack \frac{k_{m} \cdot E \cdot \left( \frac{t}{b} \right)^{2}}{\left( {1 - v^{2}} \right) \cdot \beta \cdot R_{{p0},2}} \right\rbrack^{M} \cdot R_{{p0},2}}$(Source: Schriever, T. see above).

On the one hand σ_(m) depends on the material (higher-strength steelshave significantly better σ_(m) values than conventional longitudinalmember steels), on the other hand σ_(m) also depends on the geometricshape of the longitudinal member or of the individual profiles.

It should be pointed out that the invention exclusively relates to thegeometric shape of the new longitudinal member. However, it is alsopossible to achieve further-reaching weight savings by way of selectingthe strength of the material, as has been explained in example 1 above.However, clear weight savings can also be achieved simply by selectingthe same material as used in the state of the art. We refer to example 2in this context.

EXAMPLES

Example 1

Example 1 ZStE 300, R_(p0.2) = 300 DP-K 34/60, R_(p0.2) = 300 N/mm²N/mm² T = 1.35 mm Sheet metal thickness T = 1.00 mm A = U x t = 462 mm xCross-sectional area A = 2 × 248 mm × 1.00 1.35 mm = 623.7 mm² as adimension for the mm = 496.00 mm² mass m − A = 100% Mass m = 79.5% σ_(m)= 58.1 N/mm² Average tension σ_(m) = 83.3 N/mm² F_(m) = 36.2 kN Force −absorbed F_(m) = 41.3 kN energy

Example 2

Example 1 ZStE 300, R_(p0.2) = 300 ZStE 300, R_(p0.2) = 300 N/ N/mm² mm²T = 1.35 mm Sheet metal thickness T = 1.00 mm A = U x t = 462 mm xCross-sectional area A = 2 × 248 mm × 1.00 1.35 mm = 623.7 mm² as adimension for the mm = 496.00 mm² mass m − A = 100% Mass m = 79.5% σ_(m)= 58.1 N/mm² Average tension σ_(m) = 77.6 N/mm² F_(m) = 36.2 kN Force −absorbed F_(m) = 38.5 kN energy

1-3. (Cancelled).
 4. A longitudinal member as part of a supportingstructure of a vehicle, with said longitudinal member being arranged infront of and behind the passenger compartment of the vehicle, and withsaid longitudinal member's width/height ratio being a₀/b₀<1, whereinsaid longitudinal member comprises at least two hollow sections whichextend essentially parallel to each other, wherein the hollow sectionshave a width/height ration of a_(1,2)/b_(1,2)˜1; are arranged so as tobe spaced apart from each other with a free space in-between; and areinterconnected at both ends so as to be dimensionally stable.
 5. Thelongitudinal member of claim 4, wherein the longitudinal member isarranged in front or behind the passenger compartment of the vehicle. 6.The longitudinal member of claim 4, wherein the overall height b₀ of thelongitudinal member exceeds by a multiple factor the height b_(1,2) ofeach hollow section.
 7. The longitudinal member of claim 4, whereinplates, sheet metal pieces, or combination thereof installed at theends, laterally, or combination thereof provide a dimensionally stableconnection between the hollow sections.